This invention relates generally to a multilayer film suitable for packaging, which film is preferably coextruded, and more particularly to a hot blown film suitable for use in chub packaging and having improved impact resistance both under ambient and cool conditions, interlaminar bond strength and seal strength.
Certain foods, and particularly meat products such as ground beef and other comminuted food substances, are commonly packaged in what are called chub packages. These packages are generally produced at central processing plants operated by supermarket chains or meat packing companies. These packages are generally produced using a vertical form fill seal (VFFS) process, in which a tube is formed from a flat sheet of roll stock film. The tube is formed vertically and longitudinally sealed with a vertical sealing bar. The bottom of the tube is then sealed with a clip applied to the bottom of the tube, the meat product such as ground beef is pumped into the open end of the tube, and the top is sealed with a clip to produce the final chub package. In appearance, these chubs resemble semi-rigid tubes with the tubular film forming a skin tight layer around the food product.
Package sizes may range from 1 to 20 pounds, depending on the intended mode of distribution. Pumping equipment typically used to stuff the food product into the tubular film can place great stress on the longitudinal seal of the tube. This longitudinal seal is usually a lap seal.
Crust-frozen items have been often used and the hardening of the outer surface of the food product in the tubular casing during the freezing process can produce further stress on the longitudinal seal. Thermoplastic polymers used under frozen conditions generally become more brittle than in cool storage conditions.
Successful films for use in such applications include the HS660, HS2000 and HS 3000 films produced commercially by the Cryovac company. The films are composed of lay-flat tubular film. Prior to collapse, the HS660 film includes an inner layer of ethylene vinyl acetate copolymer (EVA) and an outer layer of polyethylene (PE). The two core layers comprise nylon, with an intermediate layer between each nylon layer and respective outer layer and inner layers comprising ionomer resins. In the case of HS2000, an ethylene vinyl alcohol copolymer replaces the nylon layer closest to the inside of the structure. HS 3000 is a collapsed film and includes PA and EVOH.
U.S. Pat. No. 4,909,726 gives instructions how to make a suitable film for chub packaging. The proposed structure is a blown “collapse” structure, comprising polyamide (PA) and ethylene vinyl acetate alcohol (EVOH). Suitable materials for collapse are referred as “ethylene ester copolymers”. A “collapse” film is produced by a hot blown film joined to itself by blocking as is well known in the art. U.S. Pat. No. 4,909,726 gives a good description of this process. These structures, mentioned in U.S. Pat. No. 4,909,726, are supposed to give adequate impact strength and seal strength for the specific application. However, these structures have the following deficiencies:
1. Polyamide is a hard and tough material, but in temperatures like 0° C. or under frozen conditions, it is much more brittle than under ambient temperatures.
2. The toughness of the films depends very much on the moisture uptake, which is generally a non controllable phenomenon.
The object of the present invention is therefore to provide films particularly suitable for chub packaging in which above mentioned disadvantages are overcome.